This invention relates to a fastening device. In particular the invention relates to a fastening device which includes a fastener having a threaded shank, which in use is engaged in a threaded bore formed in a receiving member. The shank may extend through a number of intermediate members which are to be clamped to the receiving member by the fastener.
It is well known that conventional threaded fasteners, such as bolts, set screws, cap screws and the like, after having been tightly screwed into a receiving member tend to become loosened when subjected to vibration.
Many methods of overcoming this problem have been proposed, however the present invention is concerned with fastening devices which employ locking arrangements having interengaging ramps or cams located between the head of the fastener, or a nut engaged on the fastener and the receiving member. In known devices the locking arrangement may comprise a pair of annular washers, each having a cam face on one side and a friction face on the opposite side. Such a washer pair is employed cam-face to cam-face, between the head of the fastener, or the nut and the receiving member. Alternatively the head of the fastener, or the nut may carry one of the cam-faces, which interacts directly with the cam-face of a single washer. In all cases the rise or pitch of the cams is greater than the rise or pitch of the threads of the fastener.
In use, on tightening the fastener, the washer adjacent the head or nut (if present) becomes fixed thereto through the engagement of the friction surface of the washer with the surface of the nut or fastener head. Likewise the other washer becomes fixed to the receiving member (or an intermediate member if present).
When the assembly is subjected to vibration and the nut or fastener head attempts to rotate in the loosening direction, the cam face on the head, nut or washer fixed thereto, is forced to climb the cams of the washer fixed to the receiving member (or intermediate member if present), thereby increasing the clamping load and preventing the further loosening of the fastener.
Fastening devices employing this type of locking arrangement have been known for a very long time. For example arrangements of this type were described in U.S. Pat. Nos. 723,590 and 743,822 of 1903 and more recently in U.S. Pat. Nos. 3,263,727; 3,329,190 and 4,134,438; German Pat. Nos. 684466 and 2413760 and British Pat. No. 907826.
However, all of these known arrangements suffer from two major disadvantages which preclude their use in applications where stringent safety regulations are in force with regard to the possibility of fastening devices failing in use and, where the fastening devices need to be released and reused.
Firstly, when the fastener is tightened the friction face, or faces of the washer, or washers is caused to scrape across the face, or faces onto which it, or they are to become fixed. This scraping causes considerable damage to both mating components, which can result in premature failure of either and which reduces the resulting grip between the components. These problems are exacerbated by any repeated tightening and untightening of the fastener. Secondly in the course of unscrewing the fastener, the cam-faces are caused to ride over each other. Although in previous arrangements the maximum `lift` of the cams has been restricted, so as not to cause the shank of an associated fastener to be stretched beyond its elastic limit when the fastener is unscrewed; this has not proven effective in practice. It has been found that in many applications there is a grave risk of the shank of the fastener being stretched beyond its yield point and in others, the fastener and the cam-faces are damaged beyond repair when attempts are made to release the fastener.